The present invention relates generally to the purification of steam produced by geothermal sources and more particularly to a process to reduce the hydrogen sulfide content of the steam.
Geothermal steam has been used for power generation for many years and current deminishing hydrocarbon fuel reserves provide a motive for increasing the utilization of such alternate energy sources.
In general, steam recovered from underground geothermal sources includes, water, carbon dioxide, hydrogen sulfide as well as other contaminating gases. In many cases the hydrogen sulfide content of the steam is sufficiently high to make discharge of the steam into the air environmentally unacceptable. Release of the geothermal steam may occur during the drilling of a geothermal well, during the utilization of the geothermal steam in power plants, or during bleeding of geothermal wells to the atmosphere.
The removal of hydrogen sulfide from gases has been accomplished by many well known processes. For example, it is well known to remove hydrogen sulfide from gaseous stream by contacting the gas with an aqueous alkaline solution of hydrogen peroxide. However, economic considerations concerning this system may prevent full scale commercial utilization of the process.
Another known method to remove hydrogen sulfide from gases is to contact the gas with activated carbon to catalyze the oxidation of hydrogen sulfide to elemental sulfur and water. In addition, many other methods including the iron sponge process have been used to reduce the hydrogen sulfide content therein. In the latter process the gas containing hydrogen sulfide is removed by the adsorption reaction of the hydrogen sulfide with a hydrated ferric oxide as shown in the chemical reaction: EQU Fe.sub.2 O.sub.3.H.sub.2 O+3H.sub.2 S.revreaction.Fe.sub.2 S.sub.3.H.sub.2 O+3H.sub.2 O. (1)
After a period of time, the bed becomes ineffective in adsorbing any more hydrogen sulfide because the hydrated ferric oxide is no longer available for reaction. At this time the iron oxide is regenerated by passing air through the bed at ambient conditions to produce elemental sulfur as shown in the chemical equation: EQU Fe.sub.2 S.sub.3.H.sub.2 O+3O.sub.2 .revreaction.Fe.sub.2 O.sub.3.H.sub.2 O+3S.sub.2. (2)
As the iron sponge is regenerated over and over, sulfur coats the sponge and eventually forces the replacement of the sulfur coated iron sponge with fresh material.
While the use of ferric oxide has been used for reducing the hydrogen sulfide content of dry gases such as natural gas and manufactured gas or the like, which have low moisture content, the use of ferric oxide has been considered an impractical method for reducing the hydrogen sulfide content in geothermal steam because of the high water content therein.
Since the ferric oxide method has as a reaction product, water, the presence of water in the steam may impede and slow down the reaction by which hydrogen sulfide is removed. That is, with reference to equation (1) between ferric oxide and hydrogen sulfide, an equilibrium condition is reached earlier because of the presence of water in the steam thereby reducing the effectiveness and the amount of hydrogen sulfide that can be removed from the steam.
In other terms, the presence of water in the steam may promote the reverse reaction: EQU Fe.sub.2 S.sub.3 +3H.sub.2 O.fwdarw.Fe.sub.2 O.sub.3 +3H.sub.2 S (3)
Thus, there may be a competing reaction, when water is present in the gas or steam, that regenerates hydrogen sulfide thereby making the use of ferric oxide or activated carbon impractical or ineffective in removing hydrogen sulfide. This regeneration of hydrogen sulfide may be enhanced at higher temperatures as it is well known that the iron sponge process is effective in the temperature range of approximately 100.degree. F., with the reaction rate of Fe.sub.2 S.sub.3.H.sub.2 O formation (see equation (1)) dropping off at higher temperatures.
A further disadvantage of the conventional iron sponge process is the time needed to regenerate the reaction bed by passing air or oxygen therethrough. Prior art has taught the passing of an oxygen containing hydrogen sulfide to react with the Fe.sub.2 S.sub.3 to form Fe.sub.2 O.sub.3 but this has only been attempted with dry gases. Further, as hereinabove noted, the iron sponge process is effective only in the temperature range of approximately 100.degree. F. Hence, the process has not been considered as practical for removing the hydrogen sulfide from geothermal steams having temperatures of 300.degree. F., or above.